Offset suppression in micromachined Lorentz force magnetic sensor by current chopping

Abstract

Offsets (short and long term) are significantly reduced in a Lorentz force magnetometer circuit. A modulated bias current supplied to the magnetometer is chopped by periodically switching its polarity. Magnetometer output is demodulated, then de-chopping performed to restore signal polarity output. Chopping of the bias current signal polarity modulates magnetic field signal to a frequency in which electrostatic force remains constant toward eliminating offset and long-term drift from said micromechanical resonator.

What is claimed is: 
     
       1. An apparatus for magnetic force sensing, comprising:
 a Lorentz force magnetometer; 
 a first oscillator configured for oscillating at a resonant flexural frequency of said Lorentz force magnetometer; 
 a bias current signal generator configured for supplying a bias current through a resonator of said Lorentz force magnetometer modulated at said resonant flexural frequency of said Lorentz force magnetometer; 
 a drive circuit for mixing said resonant flexural frequency of said Lorentz force magnetometer with an AC bias current signal to drive a drive input of said Lorentz force magnetometer in response to output from a sense output of said Lorentz force magnetometer; 
 a second oscillator configured for oscillating at a chopping frequency; 
 a first chopper circuit configured for reversing polarity of bias current through said resonator at a chopping frequency output by said second oscillator; 
 an amplifier coupled to the sense output from said Lorentz force magnetometer; 
 a demodulator configured for receiving an amplified output from said amplifier and for demodulating that amplified output to remove said resonant flexural frequency of said Lorentz force magnetometer; and 
 a second chopper circuit for restoring signal polarity output from said magnetometer, while being driven at said chopping frequency output by said second oscillator; 
 wherein chopping of said bias current signal polarity modulates a magnetic field signal to a frequency whereby electrostatic force remains constant toward eliminating offset and long-term drift from said micromechanical resonator. 
 
     
     
       2. The apparatus recited in  claim 1 , wherein as bias current is supplied through the resonator in a first polarity, electrostatic and Lorentz forces are in-phase, while supplying bias current in a second polarity results in electrostatic and Lorentz forces which are in opposite phases. 
     
     
       3. The apparatus recited in  claim 1 , wherein said first oscillator comprises an external oscillator configured with its frequency output set to the resonance frequency of the magnetometer to drive it in an open-loop mode. 
     
     
       4. The apparatus recited in  claim 1 , wherein said first oscillator comprises said magnetometer itself in an oscillator circuit to drive said magnetometer in a closed-loop mode. 
     
     
       5. The apparatus recited in  claim 4 , wherein said oscillator circuit includes a phase-locked loop circuit, voltage limiter or a 1 bit analog to digital converter (ADC). 
     
     
       6. The apparatus recited in  claim 1 , wherein said AC bias voltage signal is an inverse-cosine function of said resonant flexural frequency of said Lorentz force magnetometer, and adjusting said AC bias voltage signal sets electrostatic oscillation amplitude. 
     
     
       7. The apparatus recited in  claim 6 , wherein frequency of said chopping frequency is substantially lower than said AC bias current signal. 
     
     
       8. The apparatus recited in  claim 7 , wherein said chopping frequency is from a few Hz to approximately tens of Hz. 
     
     
       9. The apparatus recited in  claim 1 , wherein said Lorentz force magnetometer is a micro-electromechanical system (MEMS) device. 
     
     
       10. The apparatus recited in  claim 1 , wherein said apparatus for magnetic force sensing can be utilized in an electronic compasses for determination of heading and for navigation. 
     
     
       11. The apparatus recited in  claim 1 , further comprising a low-pass filter configured for filtering output from said demodulator prior to reaching said second chopping circuit. 
     
     
       12. The apparatus recited in  claim 1 , further comprising a low-pass filter configured for filtering output from said second chopping circuit before output from said apparatus. 
     
     
       13. An apparatus for magnetic force sensing, comprising:
 a Lorentz force magnetometer as a micro-electromechanical system (MEMS) device; 
 a first oscillator configured for oscillating at a resonant flexural frequency of said Lorentz force magnetometer; 
 a bias current signal generator configured for supplying a bias current through a resonator of said Lorentz force magnetometer modulated at said resonant flexural frequency of said Lorentz force magnetometer; 
 a drive circuit for mixing said resonant flexural frequency of said Lorentz force magnetometer with an AC bias current signal to drive a drive input of said Lorentz force magnetometer in response to output from a sense output of said Lorentz force magnetometer; 
 a second oscillator configured for oscillating at a chopping frequency; 
 a first chopper circuit configured for reversing polarity of bias current through said resonator at a chopping frequency output by said second oscillator; 
 wherein as bias current is supplied through the resonator in a first polarity electrostatic and Lorentz forces are in-phase, while supplying bias current in a second polarity results in electrostatic and Lorentz forces which are in opposite phases; 
 an amplifier coupled to the sense output from said Lorentz force magnetometer; 
 a demodulator configured for receiving an amplified output from said amplifier and for demodulating that amplified output to remove said resonant flexural frequency of said Lorentz force magnetometer; 
 a low-pass filter configured for filtering output from said demodulator; 
 a second chopper circuit for restoring signal polarity output from said magnetometer, while being driven at said chopping frequency output by said second oscillator; and 
 a low-pass filter configured for filtering output from said second chopping circuit; 
 wherein chopping of said bias current signal polarity modulates a magnetic field signal to a frequency whereby electrostatic force remains constant toward eliminating offset and long-term drift from said micromechanical resonator. 
 
     
     
       14. The apparatus recited in  claim 13 , wherein said first oscillator comprises an external oscillator configured with its frequency output set to the resonance frequency of the magnetometer to drive it in an open-loop mode. 
     
     
       15. The apparatus recited in  claim 13 , wherein said first oscillator comprises said magnetometer itself in an oscillator circuit to drive said magnetometer in a closed-loop mode. 
     
     
       16. The apparatus recited in  claim 15 , wherein said oscillator circuit includes a phase-locked loop circuit, voltage limiter or a 1 bit analog to digital converter (ADC). 
     
     
       17. The apparatus recited in  claim 13 , wherein said AC bias voltage signal is an inverse-cosine function of said resonant flexural frequency of said Lorentz force magnetometer, and adjusting said AC bias voltage signal sets electrostatic oscillation amplitude. 
     
     
       18. The apparatus recited in  claim 17 , wherein frequency of said chopping frequency is much lower than said AC bias current signal. 
     
     
       19. The apparatus recited in  claim 13 , wherein said apparatus for magnetic force sensing can be utilized in an electronic compasses for determination of heading and for navigation. 
     
     
       20. A method of magnetic force sensing, comprising:
 supplying a bias current through a resonator within a Lorentz force magnetometer, said bias current being modulated at a resonant flexural frequency of said Lorentz force magnetometer; 
 mixing said resonant flexural frequency and an AC bias voltage signal to drive said Lorentz force magnetometer; 
 chopping said bias current at a chopping frequency for periodically switching polarity of bias current applied through said resonator; 
 amplifying output from said Lorentz force magnetometer; 
 demodulating amplified output to remove said resonant flexural frequency of said Lorentz force magnetometer leaving only a magnetic sensing signal; and 
 unchopping output after demodulating to restore signal polarity output from said magnetometer; 
 wherein chopping of said bias current signal polarity modulates a magnetic field signal to a frequency whereby electrostatic force remains constant toward eliminating offset and long-term drift from said micromechanical resonator.